This invention relates to a digital single sideband generator for use in a signal processor.
As is known in the art, signal processors have been used extensively in many applications such as in sonar and radar. For example, a signal processor may be used in a radar receiver for determining the Doppler frequency associated with a detected object. In such radar receiver a bipolar video signal is produced in response to each one of a train of transmitted pulses. A predetermined time after each one of the train of pulses is transmitted, the bipolar video signal is sampled to obtain a set of such signals over a relatively long period of time. The rate of change in amplitude between samples in such a set is indicative of the Doppler frequency of an object contributing to such amplitude. As is known, such a set of sampled bipolar video signals may be passed through a spectrum analyzer to determine such Doppler frequency. The number of sampled bipolar video signals in each such set is related to the Doppler frequency resolution desired. For known reasons, inter alia reliability considerations, it is desirable that the signal processor process the samples of the bipolar video signal digitally. It follows then that such signal processor should use, as far as practical, digitally operative subassemblies.
One subassembly, however, which may not be practical to implement digitally is the spectrum analyzer. That is, while it is known that fast Fourier transform (FFT) devices may be used as a spectrum analyzer, such devices are generally complex and require extensive digital circuitry. Therefore, it is sometimes more practical to use a bank of analog filters as the frequency analyzer. One difficulty in using such a bank of analog filters may exist where the range of Doppler frequencies associated with an object is relatively narrow. In order to increase the actual range of Doppler frequencies in such cases, time compression techniques have generally been used. Such time compression techniques have sometimes been implemented in accordance with U.S. Pat. No. 2,958,039 entitled "Delay Line Time Compressor," issued Oct. 25, 1960 to V. C. Anderson. Briefly, according to the just-cited patent, samples of a bipolar video frequency are taken at a relatively low rate. Successive ones of the samples are coupled directly to a recirculating delay line having a time delay so related to the rate at which the samples are taken that the output signal from such delay line ultimately is made up of successive samples at a much higher rate. Consequently, the frequency spectrum of such output signal is increased correspondingly relative to the frequency spectrum of the bipolar video signal coupled to such time compressor. Therefore, the use of a bank of analog filters is made practical. It is also known that the output signal from the recirculating delay line may be heterodyned with a frequency-varying signal produced by a frequency selector network to separate the frequency components in such output signal for analysis by the bank of analog filters. The signal produced by the frequency selector changes frequency in steps cyclically over a predetermined period of time. In this way, the bank of analog filters may be used on a "time shared" basis to analyze Doppler frequencies over a band of frequencies larger than the bandwidth of the bank of analog filters. The frequency selector network sometimes used includes a voltage controlled oscillator driving a single side band generator, the output of such single side band generator being the frequency-varying signal to be heterodyned with the output signal from the recirculating delay line. For reasons mentioned above it would be desirable, however, not to require the use of a completely analog processor but rather to permit a frequency selector network to be made up of digitally operative subassemblies, i.e. a digitally controlled oscillator and a digitally operative single side band generator.
In a known radar system wherein a Doppler frequency resolution requirement dictates the use of a set made up of 640 samples of the bipolar video signal, such samples occurring at a rate of about 10 KHZ, a time compressor mechanized according to U.S. Pat. No. 2,958,038 would require a delay line (for example a digital shift register) operating at a 6.4 MHZ rate. Known delay lines adapted to operate at such 6.4 MHZ rate are, however, relatively costly and complex.